This invention relates to a method and an apparatus for identifying the location of information on a magnetic tape such as the type that may be used in a video cassette tape recorder (VCR). The method and apparatus may be employed in a wide range of devices for recording or playback of audio or visual information ranging from home video cassette and audio tape recorders to automatic information archiving and retrieval systems.
Presently, in a video cassette recorder, a magnetic tape is encased in a cassette having a mechanical form factor that has become standardized throughout the world. For a variety of reasons it is desirable to provide a visual or audible indication of the location of specific information on the tape. For example, in a VCR, it may be desirable to locate a scene in a movie, that was previously identified during normal operation of the VCR through operation of the fast forward or reverse movement of the tape. One common method for determining the location of information on a tape is through a mechanical counter. As a tape is wound or rewound, rotation of the spool is mechanically counted with the information provided through wheels similar to an odometer on an automobile. One problem with this approach is the complexity of converting revolutions to tape location since the diameter of the tape on the spool varies with each revolution. To overcome this drawback, a separate counter wheel may be movably mounted in frictional contact with the tape. But another problem with a mechanical counter is that each time a tape is begun, the counter must be reset to zero to determine the reference from which a location on the tape may later be determined. If a different machine is used, the counter must be the same as on the the machine originally used or the desired location will be mis-identified. Furthermore, in the event that a location is desired to be found on a tape that has not been completely rewound, it would be necessary to completely rewind the tape so as to reset the counter to provide the reference. This is not only time consuming and creates excessive wear on the tape, but is also an inconvenience. Furthermore, a typical magnetic tape used in video cassettes is temperature sensitive resulting in stretching or shrinkage. This shrinkage adversely affects the accuracy of the location-determining counter. Also, permanent stretching results as the tape is pulled through the playback machine during normal operation. After repeated use of the tape, such stretching can result in significant errors in a mechanical counting mechanism. Furthermore, mechanical counters may vary from machine to machine. Different manufacturers may use different mechanisms that would result in different results. Since there are typically no holes in video cassette magnetic tape, wheel slippage and difference in the counting wheel radius caused by wear or manufacturing tolerances will further result in errors regarding accurate location. The result is inaccuracy rendering the mechanical tape counters unsuitable for uses where precise positional information is required.
As an alternative to mechanical counters in the prior art, attempts have been made to electronically or magnetically record codes onto the tape which can then be read by various methods. Generally, the difficulty with these methods is that the recorded information that indicates the location of information on the tape may be easily erased in the event that the tape is re-recorded. Moreover, a typical video cassette tape that is prerecorded is write-protected which would prevent any manufacturer other than the one recording the information on the tape from placing the position-indicating codes on the tape. A particular drawback of magnetic recording of codes on the tape is that they can be read only when the tape is in physical contact with the magnetic read head. In the typical video cassette tape, when in fast forward or rewind modes, the tape is not in contact with the read head. Even if the cassette and playback machines in current use were to be abandoned, so that the read head was in contact with the tape during rewinding or fast forward, it would cause excessive wear on the tape and head during search operations. Moreover, even if the read head was in contact with the tape, the speed of the tape during fast forward and rewind would make accurate location reading difficult or unreliable using the electronic technique in use in most VCR's today.
There are also instances in the prior art, particularly with respect to microfilm and photographic film reading devices, where bar codes are placed on strips of film, permitting a machine-specific optical read-out that relates location indicia with specific optical information on the strip, e.g. a frame on a microfilm tape. This allows a mechanism to retrieve a specific frame on a strip by identifying its associated bar code rather than to locate a specific position on the strip itself. Furthermore, these optically-read bar codes on the film are applied simultaneously with the imprinting of the optical information on the film since it is necessary that the recording mechanism be able to record not only the desire information but the bar code information as well.
Finally, as a result of the popularity of video cassette recording, there are now millions if not billions of cassettes with prerecorded information that are in widespread use. Any mechanism or method for determining the position of information on the tape must be compatible with existing video cassette mechanical structure. Ideally, the method and apparatus for reading the location of information on the tape is also desirably retrofitted to existing VCR equipment.